Gas turbine engines, particularly those which power aircraft, are typically provided with accessories such as electrical generators, pumps and the like, which are required for operation of the engine and an associated aircraft. It is common practice to drivingly engage such accessories to the engine by means of an accessory gearbox which is itself drivingly engaged to high pressure shaft of the engine, so as to be able to drive the high pressure shaft and the accessories during engine start-up. However, the drive of the accessories have an impact on the fuel efficiency of the engine.